1. Field of the Invention
The present invention relates to an ultrasonic measurement method and an ultrasonic measurement apparatus. The invention more particularly relates to an ultrasonic measurement method and an ultrasonic measurement apparatus, which are suitable to use phased array transducer (array sensor) having ultrasonic transducer elements that are two-dimensionally arranged, and to the ultrasonic sensor.
2. Description of the Related Art
In a conventional technique, when it is difficult to predict a direction in which a defect will become larger, the defect may be overlooked in an ultrasonic inspection process for transmitting an ultrasonic wave in an axial direction and a circumferential direction. Thus, there is demand for an ultrasonic inspection process for transmitting an ultrasonic wave in all directions. Since this type of defect is detected by an angle beam inspection process, an ultrasonic inspection technique that simultaneously achieves those inspection processes is essential. In addition, in order to respond to demand for a reduction in an inspection time, it is necessary to develop a technique that is capable of performing an ultrasonic inspection process for transmitting an ultrasonic wave in all directions without mechanical scanning.
On the other hand, when a fixed-angle sensor is used, mechanical rotary scanning needs to be performed. In this case, in order to change an incident angle of an ultrasonic wave, the sensor needs to be replaced. Thus, it takes a long time for a measurement. A linear array sensor having ultrasonic transducer elements that are one-dimensionally arranged is capable of performing two-dimension-like scanning by means of a phased array technique in which the timing for applying an electric signal to each ultrasonic transducer element is controlled so that an ultrasonic beam converges at any refraction angle in a direction in which the elements are arranged. Thus, it is not necessary to replace the sensor. However, in order to perform a measurement in all directions, mechanical rotary scanning needs to be performed for the same reason as in the measurement using the fixed-angle sensor. Therefore, it takes a long time for the measurement.
In recent years, a phased array technique that uses a matrix array sensor having ultrasonic transducer elements two-dimensionally arranged has been actively researched and developed in order to reduce the measurement time (for example, refer to JP-A-2005-351718). In this method, it is possible to three-dimensionally transmit and receive an ultrasonic wave by controlling the timing for applying an electric signal in a manner similar to that in the linear array sensor; therefore, it is possible to perform a measurement in all directions for a short time without performing mechanical rotary scanning while obtaining a good signal-to-noise (SN) ratio.
The present inventors have studied on a three-dimensional ultrasonic technique that uses a matrix array sensor to inspect a plate material having a large thickness.
It is necessary that a sensor aperture through which an ultrasonic wave is transmitted and received be larger in order to determine whether or not a crack exists in a deep portion of the plate material in an inspect method using the three-dimensional ultrasonic technique.
When the size of a sensor aperture of a matrix array sensor constituted by ultrasonic transducer elements whose areas are approximately the same is increased in a manner similar to that in a conventional rectangular matrix sensor or a segmented sensor array, the number of ultrasonic transducer elements is increased in proportion to the square of the size of the sensor aperture.
However, the number of the ultrasonic transducer elements that can be controlled by a phased array apparatus is restricted for a technical reason. In addition, even when the number of the ultrasonic transducer elements that can be controlled by the phased array apparatus is increased, the apparatus would be larger and more expensive; therefore it is difficult to increase the size of a sensor aperture. In addition, performing a measurement in all directions at 360 degrees using a conventional rectangular matrix array sensor may cause a variation in sensitivity due to symmetry of the arrangement of ultrasonic transducer elements constituting the array sensor or due to the shapes of the ultrasonic transducer elements.
To avoid the above problems, a method is known which selects, from among ultrasonic transducer elements that are two-dimensionally arranged and constitute an array sensor, only a group of ultrasonic transducer elements that are arranged in a direction in which electronic scanning is performed and uses the selected elements to transmit and receive an ultrasonic wave (for example, refer to JP-5-244691-A).
In addition, another method is known which selects a transmitting element and a receiving element in a zigzag manner from among ultrasonic transducer elements that constitute an array sensor and are two-dimensionally arranged and uses the selected elements to transmit and receive an ultrasonic wave (for example, refer to JP-2003-599-A).